The current ability to test theories of gravity with black hole shadows

Astrophysicists at Frankfurt, the Max Planck Institute for Radio Astronomy in Bonn, and Nijmegen, collaborating in the project BlackHoleCam, answer this question by computing the first images of feeding non-Einsteinian black holes: it is presently hard to tell them apart from standard black holes.

One of the most fundamental predictions of Einstein's theory of relativity is the existence of black holes. In spite of the recent detection of gravitational waves from binary black holes by LIGO, direct evidence using electromagnetic waves remains elusive and astronomers are looking for it with radio telescopes. For the first time, collaborators in the ERC funded project BlackHoleCam, including astrophysicists at Goethe University Frankfurt, Max Planck Institute for Radio Astronomy (MPIfR) Bonn, and Radboud University Nijmegen, have compared self-consistent and realistic images of the shadow of an accreting supermassive black hole – such as the black-hole candidate Sagittarius A* (Sgr A*) in the heart of our Galaxy – both in general relativity and in a different theory of gravity. The goal was to test if Einsteinian black holes can be distinguished from those in alternative theories of gravity.

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Credit: Fromm/Younsi/Mizuno/Rezzolla (Frankfurt)

Synthetic shadow images of Sgr A* for a Kerr black hole (top row) and a non-rotating dilaton black hole (bottom row). In each case the left panel refers to the image produced by the general-relativistic magnetohydrodynamic simulations, while the right panel refers to the image reconstructed after realistic observational conditions are considered.

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A black hole has such an enormous mass and gravitational force that it essentially collapses in on itself.

 

Animation of the shadow (outlined in red) of a maximally spinning (Kerr) black hole. The event horizon (blue) of the black, interior to the shadow, is unobservable.

The observer starts by looking at the black hole from above (i=0 degrees), moving towards the equator and looking at it edge-on (i=90 degrees). The shadow is initially circular, but becomes more 'squashed' as the observer's viewing angle increases.

If the black hole were not spinning (Schwarzschild) the shadow would not change shape at all. In principle, measurement of the size and shape of this shadow allows us to extract information about the mass, spin and relative orientation of the black hole.

 

This simulation shows a star getting torn apart by the gravitational tides of a supermassive black hole. The star gets “spaghettified” and after several orbits creates an accretion disc. Scientists believe that the superluminous ASASSN-15lh event originated in this way. The view on the right is from the side and that at the left face on.